feat(demo): add landing page with docs (#1951)

docs/app/@core/data/service/header-menu.service.ts

@@ -0,0 +1,23 @@
+import { Injectable } from '@angular/core';
+import { of as observableOf,  Observable } from 'rxjs';
+
+import { NbMenuItem } from '@nebular/theme';
+
+@Injectable()
+export class HeaderMenuService {
+
+  private headerMenu: NbMenuItem[] = [
+    {
+      title: 'Home',
+      link: '/',
+    },
+    {
+      title: 'Docs',
+      link: '/docs',
+    },
+  ];
+
+  getHeaderMenu(): Observable<NbMenuItem[]> {
+    return observableOf(this.headerMenu);
+  }
+}